Multiply.
Solution: When a number is next to a variable, it means they are multiplied. So, $({-1}{b^7})({7}{b^4})$ is the same as $({-1})({b^7})({7})({b^4})$. Now we can rearrange the factors because multiplication is commutative (a fancy way of saying that the order in which we multiply things doesn't matter). ${(-1)(7)}{(b^7)(b^4)}$ Then simplify, and we're done! ${-7}{b^{11}}$